Harpers

PLASMA PROTEINS & IMMUNOGLOBULINS / 581Table 50–1. Major functions of blood.(1) Respiration—transport of oxygen from the lungs to thetissues and of CO 2 from the tissues to the lungs(2) Nutrition—transport of absorbed food materials(3) Excretion—transport of metabolic waste to the kidneys,lungs, skin, and intestines for removal(4) Maintenance of the normal acid-base balance in thebody(5) Regulation of water balance through the effects ofblood on the exchange of water between the circulatingfluid and the tissue fluid(6) Regulation of body temperature by the distribution ofbody heat(7) Defense against infection by the white blood cells andcirculating antibodies(8) Transport of hormones and regulation of metabolism(9) Transport of metabolites(10) CoagulationPlasma Proteins Have BeenStudied ExtensivelyBecause of the relative ease with which they can be obtained,plasma proteins have been studied extensively inboth humans and animals. Considerable information isavailable about the biosynthesis, turnover, structure,and functions of the major plasma proteins. Alterationsof their amounts and of their metabolism in many diseasestates have also been investigated. In recent years,many of the genes for plasma proteins have been clonedand their structures determined.The preparation of antibodies specific for the individualplasma proteins has greatly facilitated theirstudy, allowing the precipitation and isolation of pureproteins from the complex mixture present in tissues orplasma. In addition, the use of isotopes has made possiblethe determination of their pathways of biosynthesisand of their turnover rates in plasma.The following generalizations have emerged fromstudies of plasma proteins.Albumin69,000β 1 -Globulin90,000α 1 -Lipoprotein200,00010 nmScaleNa + CI –GlucoseHemoglobin64,500γ-Globulin156,000β 1 -Lipoprotein1,300,000A. MOST PLASMA PROTEINS ARESYNTHESIZED IN THE LIVERThis has been established by experiments at the wholeanimallevel (eg, hepatectomy) and by use of the isolatedperfused liver preparation, of liver slices, of liverhomogenates, and of in vitro translation systems usingpreparations of mRNA extracted from liver. However,the γ-globulins are synthesized in plasma cells and certainplasma proteins are synthesized in other sites, suchas endothelial cells.B. PLASMA PROTEINS ARE GENERALLY SYNTHESIZEDON MEMBRANE-BOUND POLYRIBOSOMESThey then traverse the major secretory route in the cell(rough endoplasmic membrane → smooth endoplasmicmembrane → Golgi apparatus → secretory vesicles) priorto entering the plasma. Thus, most plasma proteins aresynthesized as preproteins and initially contain aminoterminal signal peptides (Chapter 46). They are usuallysubjected to various posttranslational modifications (proteolysis,glycosylation, phosphorylation, etc) as they travelthrough the cell. Transit times through the hepatocytefrom the site of synthesis to the plasma vary from 30 minutesto several hours or more for individual proteins.Fibrinogen340,000Figure 50–1. Relative dimensions and approximatemolecular masses of protein molecules in the blood(Oncley).C. MOST PLASMA PROTEINS ARE GLYCOPROTEINSAccordingly, they generally contain either N- or O-linked oligosaccharide chains, or both (Chapter 47). Albuminis the major exception; it does not contain sugarresidues. The oligosaccharide chains have various functions(Table 47–2). Removal of terminal sialic acid